1. Field of the Invention
This invention relates generally to an automatic focussing system, and more particularly to an automatic focussing (hereinafter called "AF") system suitable for those AF system apparatuses which effect automatic focus detection by utilizing video signals (image pickup video signals) outputted from a video camera.
2. Description of the Related Art
As an AF system which makes the most of the characterizing features of video cameras, a so-called "mountain climbing servo system" AF system which detects fineness of a picture surface by high band components in video signals, drives a focussing lens so as that the fineness attains maximum, and automatically adjusts an optical focus of the television camera has been drawing an increasing attention in recent years. This mountain climbing servo system (hereinafter called the "mountain climbing system") is described in detail in, for example, "NHK Technical Research", 1965, Vol. 17, No. 1, Series No. 86, pp. 21-37. The content of this system will be explained briefly with reference to FIG. 12 of the accompanying drawings which is a structural view of an automatic focussing system by the conventional mountain climbing system and FIG. 13 which shows the operation characteristics of the apparatus.
First of all, the rays of light from an object 1 which are incident through a focussing lens 2 form an image on photoelectric conversion means 3 and outputted as electric signals. The output signal as a video signal outputted from the photoelectric conversion means is inputted to a processing circuit 5 for a video camera through an amplifier 4 and at the same time, only high band components in the video signal are extracted by a band-pass filter (BPF) 6 juxtaposed with the processing circuit 5 at a next stage of the amplifier 4. The high band components are inputted to a gate circuit 7 of a next stage.
A window pulse forming circuit (WINDOW) 8 generates a sync pulse of only a predetermined region of one picture surface, such as only of the center of the picture surface or in other words, a so-called "window pulse", from the horizontal and vertical sync signals HD and VD separated by the processing circuit 5 for the camera described above, and the resulting window pulse is inputted to the gate circuit 7 described above in order to extract and output the signal of the high band component only during the generation period of this window pulse. The signal of the high band component outputted from the gate circuit 7 is further processed by a detector (DET) 9 and an integrator 10.
FIG. 13 shows the relationship between a voltage corresponding to the output of this integrator 10 (hereinafter called a "focal voltage") and the focal length of the lens 2. Since this focal voltage correspond to the fineness of the image which is picked up, the focal voltage becomes maximal if the position of a range ring (focus ring) for moving and adjusting the position of the focussing lens 2 is in correct agreement with the actual distance between the lens 2 and the object 1, and drops as the position deviates from this position C where the focal voltage is maximal.
Therefore, as can be seen from FIG. 13, automatic focussing can be attained by controlling the position of the lens range ring by any control means in such a manner as to climb the mountain of the focal voltage and guiding the lens range ring to the top of the mountain where the focal voltage is maximal. One of the known control means for accomplishing this object holds the output of the integrator 10 for each field of the video signal, compares the value held previously with the value held this time for each field and drives the lens range ring in a direction where the value is great. In other words, as shown in a mountain climbing circuit 16 encompassed by dash line in FIG. 12, sample pulses are generated at a predetermined timing by a monomultiple vibrator (MM) 12 and a sample pulse generation circuit (S.P) from the vertical sync signal VD separated by the camera processing circuit 5 described already, and the output of the integrator which is inputted to a sample-hold circuit (S/H) 11 is sampled and held in accordance with this sample pulse for each field. Furthermore, the output of the sample-hold circuit 11 is divided into two signals. One is as such applied to a comparator 15 while the other is applied to the comparator 15 after being delayed by one field by a one-field delay circuit (one-field delay) 14. The comparator 15 determins the difference between them, that is, the output of the integrator of the previous holding operation with that of the holding operation this time.
Suppose that the focal voltage of the output of the integrator 10 is at the level A in the previous field as shown in FIG. 13 and at the level B in a next field, the focal voltages A and B have the relation B&gt;A and hence the comparator 15 outputs a high level control signal, for example, to a motor driving circuit (DRIVER) 17 in order to drive a motor (Mo) 18 while keeping the existing driving direction. As the motor 18 is thus operated, the lens 2 is moved in a focussing direction. Suppose the focal voltage is found to be at the level C shown in FIG. 13 by detection in the next field, the levels C and B have the relation C&gt;B so that the signal level from the comparator 15 does not change and the motor 18 keeps operating in the same direction.
It will be assumed at this time that the maximum point of the focal voltage is at the level C as shown in FIG. 13. Then, if the lens range ring is kept moved as such in the same direction by the motor 18, it moves away from the focussing state C and the focal voltage tends to drop. As a result, a focal voltage of the level D is outputted in the next field and since D&lt;C, the comparator 15 outputs a low level signal to the control circuit 17. Receiving this low level signal, the control circuit 17 generates a control signal which rotates reversely the motor 18 and the lens range ring is again sent reversely in the focussing direction.
As described above, the conventional mountain climbing system shown in FIGS. 12 and 13 keeps the rotating direction of the motor 18 as such to continue mountain climbing when the output of the mountain climbing circuit 16 consisting of the comparator 15 and the like is positive (high level), that is, when the focal voltage is in the increasing direction with the passage of time, and rotates reversely the motor 18 to climb the mountain when the output of the mountain climbing circuit is negative (low level), that is, when the focal voltage is in the decreasing direction with the passage of time. Therefore, mountain climbing is made on the mountain such as of FIG. 13 formed by the focal voltage with reference to the output voltage of the integration circuit 10 and then reaches the steady state while minutely oscillating at the top of the mountain, thereby effecting automatic focus detection.
Since the mountain climbing system described above makes automatic focus detection by use of the output signal itself of image pickup means, the system has the advantages in that constituent elements for automatic focus detection need not be disposed particularly, and the system is relatively economical and can detect focus correctly.
In accordance with the conventional mountain climbing system described above, however, it is not possible to judge whether or not the top (maximum value) of the mountain of the focal voltage under the in-focus condition has been reached unless descension is made. For this reason, the steady state is reached while minute oscillation is being made near the top of the mountain. In other words, the motor repeats normal and reverse rotation under the focus state and ascension and descension are made near the top of the mountain under the constantly oscillating state. This oscillation results in degradation of picture quality and must be eliminated.